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EPHRATA, Wash. (AP) — A 65-foot-long crack in a Columbia River dam in central Washington has prompted officials to begin lowering the water level by 20 feet so inspectors can get a better idea of how serious the damage is.

There's no immediate threat to public safety from the crack in the Wanapum Dam, Grant County Public Utility District spokesman Thomas Stredwick said Friday. The dam is located just downstream from where Interstate 90 crosses the river.

"At this point we already know there's a serious problem," Stredwick said. "We want to make sure the spillway is stable enough that inspectors are safe when inspecting it."

An engineer earlier this week spotted a slight "bowing" above the spillway gates near where cars can drive across the dam, The Seattle Times reported (http://is.gd/aE0vDj ). Divers found a 2-inch-wide crack along the base of one of the spillway piers.

Public utility district officials analyzed the divers' data and decided Friday that the failure risk was sufficiently high that they should notify other government agencies and downstream water users.

Officials have lowered the water level by 6 feet already and plan to let the level drop another 14 feet by Monday.

Dam failure in the rural area south of the small town of Vantage would primarily affect farmers, fishermen and power generation. The dam can generate more than 1,000 megawatts of hydroelectric power.

PUD officials are working with the Federal Energy Regulatory Commission to determine how best to repair the cracked pillar.

Repairs could also affect the rest of the Columbia River hydroelectric system.

"All these dams coordinate to generate energy on a regional scope," Stredwick said. "If Wanapum is impacted, that has impacts on dams upstream as well as below."

Officials with the Bonneville Power Administration, the federal utility that sells and transmits much of the Northwest's cheap and abundant hydroelectric power, declined to comment on any potential impact to power generation, The Times reported.